[flang] [llvm] [flang] Add HOSTNM runtime and lowering intrinsics implementation (PR #131910)

[Eugene Epshteyn via llvm-commits]

eugeneepshteyn wrote:

> I noticed that LNBLNK seems to be in the middle of SECONDS. Might be nice to have the intrinsics in alphabetical order, if you get into this file again soon.

I will do that as a separate NFC patch, so that not to add unrelated diffs to this PR.

https://github.com/llvm/llvm-project/pull/131910